Engineers are constantly seeking strategies for reducing engine emissions. One method of reducing engine emissions is to control numerous fuel injection variables, such as fuel pressure, spray pattern, droplet size, number of injections and injection timing. For instance, it has been found that multiple injections, including pilot and post injections, during a single combustion event reduce emissions. In order to provide multiple injections during a single combustion event with reliable consistency, a fuel injector must often have the ability to reset an intensifier piston quickly while not adversely affecting the desired injection variables.
In order to reduce the amount of time the intensifier piston needs to retract while not affecting the rate shape of the injection caused by the advancing intensifier piston, fuel injectors, such as Caterpillar HEUI™ B unit injector, include a variable flow rate valve positioned in a rate shaping path that fluidly connects a hydraulic surface of the intensifier piston with either a source of actuation fluid or a low pressure drain. Such an injector is shown and described in co-owned U.S. patent application Ser. No. 10/185,946, now U.S. Pat. No. 6,663,014. In order to achieve the desired injection rate shape, the variable flow rate valve restricts the actuation fluid flow in the direction toward the intensifier piston during intensifier piston advancement. In order to quickly reset the intensifier piston, the variable flow rate valve unrestricts the actuation fluid flow in a reverse direction away from the intensifier piston during intensifier piston retraction. The variable flow rate valve includes a disc-shaped valve member with a central passage having a predetermined flow area. The flow of actuation fluid towards the intensifier piston acts to keep the valve member in contact with its valve seat, and restricts the flow of actuation fluid to the central passage of the valve member. When the intensifier piston is retracting, the flow of actuation fluid away from the piston and towards the drain lifts the member off the valve seat and allows the actuation fluid to flow through the central passage of the valve member and also around the sides of the valve member.
Although the variable flow control valve may lessen the time required to reset the intensifier piston, there is room for improvement. Engineers have discovered that the flow control valve member may not always rest against the flat valve seat when the actuation fluid is not acting on the closing hydraulic surface. Thus, when the hydraulic pressure within the rate shaping path builds as the actuation fluid begins flowing toward the intensifier piston in order to advance the intensifier piston, the hydraulic pressure may slam the variable flow rate valve member into contact with the valve seat. The repeated impacts between the valve member and the valve surface can cause the valve member and/or valve seat to wear, which can eventually cause the valve to be unable to fully close when seated. Thus, actuation fluid might eventually flow around the valve member even when the valve member is in the seated position. Because more actuation fluid than desired will be flowing past the worn valve member and acting on the intensifier piston, the injector with the worn valve member or seat will create an injection with characteristics, such as a quantity and rate shape, different than originally desired.
Further, the hydraulic pressure of the actuation fluid acting on the closing hydraulic surface can be uneven, causing the variable flow rate valve member to slant, or even bounce, within the guide bore of the rate shaping path. When the variable flow rate valve member is not aligned within the guide bore, the high pressure actuation fluid can flow around the valve member and through the central passage. Moreover, the actuation fluid flowing through the passages of the slanted variable flow rate valve member can create wear on the variable flow rate valve member. Thus, the flow area through and around the slanted variable flow rate valve member will be different than the flow area around an aligned variable flow rate valve member.
In addition to valve member wear caused by the hydraulic pressure acting on the closing hydraulic surface, wear may be caused to the edges of the valve member by actuation fluid flow around the valve member. The actuation fluid flow around the outer edges of the valve member during the intensifier piston retraction can eventually round the ninety degree corners of the variable flow rate valve member, and thereby alter the flow area around the variable flow rate valve member. The change in the shape of the flow area defined by the valve member over time may also cause a change in the injector performance over time.
Moreover, engineers have found that cavitation within the passages through and around the variable flow rate valve member can also cause wear on the variable flow rate valve member and/or valve seat. Again, the wear on the variable flow rate valve member can alter the flow area around and through the variable flow rate valve member, which in return alters the rate shape and quantity of the injection. Thus, due to variable flow rate valve wear, the ability to control injection variables with consistency and predictability is reduced, and emission reductions can be adversely affected.
The present invention is directed at overcoming one or more of the problems set forth above.